Well logging apparatus



May 9, 1961 A. H. FRENTROP 2,983,820

WELL LOGGING APPARATUS Filed March 8, 1954 4 Sheets-Sheet 1 HIS ATTORNEY May 9, 1961 A. H. FRENTROP WELL LOGGING APPARATUS 4 Sheets-Sheet 2 Filed March 8, 1954 IIIIIIIIIW l FIG.2B

FIG.2A

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INVENTOR. ARTHUR H.FRENTROP HIS ATTORNEY May 9,` l961 A. H. FRENTROP 2,983,820

WELL LOGGING APPARATUS Filed March 8, 1954 4 Sheets-Sheet 3 237 FIG 7 FIG. 8

INVENTOR. ARTHUR H.FRENTR0P HIS ATTORNEY May 9, 1961 A. H. FRENTROP 2,983,820

' WELL LOGGING APPARATUS Filed MaIOh 8, 1954 4 Sheets-Sheet 4 PRESSURE l/lqe com-Rol.

CIRCUIT www BY MMC/44M HIS ATTORNEY WELL LOGGING APPARATUS Arthur H. Frentrop, Ridgefield, Conn., assigner, by mesne assignments, to Schlumberger Well Surveying Corporation, Houston, Tex., a corporation of Texas Filed Mar. 8, 1954, Ser. No. 414,839

23 Claims. (Cl. Z50-84.5)

This invention relates to apparatus for well logging and more particularly pertains to a new and improved neutron generator especially adapted to traverse the narrow coniines of a well or borehole, although useful in a variety of other applications. Since a neutron generator embodying the invention is ideally suited to the needs of well logging service, it will be described in that connection.

It has been proposed heretofore that a generator of high energy neutrons be employed in neutron-gamma ray or in neutron-neutron logging. As contrasted with a radiumberyllium source conventionally utilized for such logging, a neutron generator may feature a negligible amount of radiation other than the desired neutrons, a higher yield of neutrons, a controllable yield of neutrons, neutrons at higher energies than formerly possible, mono-energetic neutrons and control of the generator so as to permit its deactivation prior to withdrawal from a well. The first live of these attributes are important in obtaining more informative logs, while the last is valuable in minimizing health hazards to operating personnel.

In general, prior neutron generators were only suited for laboratory use and were not designed to meet the severe requirements imposed on well logging equipment. Thus, presently available neutron generators are much too large to be passed through a borehole. The components are not adaptable to the source-detector spacing requirements of well logging. Moreover, these neutron generators are too critical in their operation and too fragile for logging service.

lt is, therefore, a primary object of the present invention to provide an improved neutron generator which meets all requirements of logging service.

A particular object of the present invention is to provide an improved neutron generator which is small enough to permit its introduction into an inherently cylindrical borehole.

Another object of the present invention is to provide an improved neutron generator which may be reliably operated during a logging run without requiring critical and continuous operating adjustments.

An additional object of the present invention is to provide an improved neutron generator which may be reliably operated at the high ambient temperatures encountered at depth in logging operations.

Yet another object of the present invention is to provide an improved neutron generator that is rugged enough to operate etiiciently and reliably although subjected to the severe physical shocks usually imposed on logging apparatus during transport to and from a well location as well as during a logging run.

These and other objects of the present invention are obtained by providing a neutron generator comprised of an ion source, an ion accelerator and a target which preferably may be an element of the accelerator. The target includes a substance adapted to react with bombarding ions of sufhcient velocity to produce neutrons.

More specifically, the generator comprises an envelope containing a gas such as deuterium. A coil encloses a Patented May 9, 1961 portion of the envelope and is energized with radiofrequency energy. The resulting radiofrequency field components excite the gas within the envelope so that a continuous ionic discharge occurs and the portion of the generator `thus far described operates as a source of ions. A probe of the accelerator, which effectively reaches into the reg-ion of the ion source, removes positive ions from the source and a suitably high potential difference is impressed between the probe and the target so that these positive ions are accelerated to the required high velocity prior to striking the target. The target includes a material containing tritium, an isotope of hydrogen. From the resulting deuterium-tritium reactions, neutrons are derived.

In a particular embodiment of the present invention, means are provided for replenishing tritium in the target. To this end, the target is constructed of the material through which tritium ions may `diffuse and is arranged to enclose and form one electrode of an electrolytic cell. The electrolyte is arranged so that tritium ions are formed in solution and by applying an electric potential between the first-mentioned electrode and another electrode of the cell, ionic migration is caused to occur. By suitably arranging the polarity of the applied potential, tritium collects at the first-mentioned electrode in ionic form and dilfuses through the target material so that it is exposed to incident deuterium ions which are accelerated ytoward the target.

In another embodiment of the invention, the target of the neutron generator is arranged so that a portion o-f the envelope between the probe and the target is reduced in diameter to form a channel that is coaxial With an opening in the probe. The target may be of toroidal configuration and axially aligned with the channel, and may have an inner diameter larger than the diameter of the channel. Alternatively, instead of the toroidal target, the channel may have a section dis posed at an angle relative to the axis dened by the opening in the probe, and a target member is disposed at the end of this section. In either of these target arrangements, a normal to the target face is not coaxial with the axis along which ions are initiated and secondary electron currents are minimized.

Alternatively, a neutron generator embodying the present invention may comprise an electron-emitting filament or cathode surrounded by a perforated electrode. The cathode and perforated electrode are suitably energized to achieve an ionic discharge for producing deuterium ions. The perforated electrode effectively functions as a probe and is surrounded by a tritium-containing target and a potential for accelerating positive ions is applied to the accelerating gap dened by the probe and the target.

In order to maintain the neutron iiux emanating from the neutron generator within prescribed limits, a detectorintegrator may be employed to derive a control potential representing a characteristic of the neutron ux, such as the number of neutrons counted per unit time. This potential may, for example, be employed to adjust the potential applied to the accelerating gap. Since the yield is dependent upon the energy of the positive ions incident on -the tritium target, automatic control of the neutron output is obtained.

The apparatus may further include a pressure-control System for maintaining the pressure of deuterium gas in the ion source at a preselected value despite the fact that ions of the gas are continuously withdrawn. For this purpose, a pressure transducer may be associated with the ion source for deriving a pressure-control potential representing the gas pressure. This potential automatically adjusts the amount of gas issuing from a deuterium supply associated with the ion source.

yThe pressure transducer may, for example, comprise a portion of the aforementioned envelope. A pair of diametrically-opposed tubes extend through the envelope and are terminated by spaced, parallel cathode plates. Bach tube receives a bar magnet to provide a magnetic field oriented transversely to the cathode plates. This field effectively increases the path for electrons travelling between the cathode plates and an annular anode member supported between the plates. By suitably energizing the anode and cathode electrodes, a continuous ionic discharge occurs wherein the resulting anode-cathode current is dependent upon the pressure of the gas, and from this current, the aforementioned pressure control potential is derived.

Pressure within the envelope may be controlled by means of a iilament-type deuterium source composed of a metal in which deuterium gas is occluded. Alternatively, a material in which deuterium is absorbed may be heated by a non-absorbent lament.

A modied deuterium source comprises an anode having a `d uteriurnabsorbent surface. ode is associated with the anode to constitute an electron discharge device whose anode-cathode current may be controlled. Accordingly, the anode dissipation may be 'adjusted and in this way the amount of deuterium gas which issues from the anode is regulated.

An alternative type of pressure control system comprises a deuteriumsource having a suiiicient capacity to provide -a pressure equilibrium. For example, at a given operating temperature, if the deuterium pressure within the envelope drops below a desired limit, the source emits deuterium; whereas, if the pressure increases above a given value, the source absorbs deuterium.

Because the neutron generator is a closed contiguous system, it is necessary to balance two opposing requirements. The gas pressure in the ion source must be high enough to Iallow suicient ionization to be produced in each to give adequate ion currents for their operation. However, the gas pressure must be low enough to avoid appreciable production of ionization in the accelerating gap. A stable balance may be achieved by constructing these two components so that the total electron path in the ion source is large compared to the Itotal electron path in the accelerating gap.

As used herein, the term mean-free-path denotes the average distance that electrons travel in a particular gas between collisions with atoms or ions of that gas. If a sufficient number of such collisions take place, the ionization produced is cumulative, resulting in a continuous ionic discharge.

The spacing between electrodes of the accelerating gap is made small enough to minimize the path travelled by electrons and ions in this region. In this way, ionization is inhibited despite the extremely high acceleration potential applied to the gap.

The arrangement in the ion source is such that the path of travel of electrons is long enough to assure the occurrence of strong ionization. This is accomplished by providing an envelope diameter for the ion source that is large enough to permit electrons which traverse vcircular paths due to the applied radiofrequency eld to travel distances longer than the mean-free-path.

The novel features of the present invention are set forth with particutlfarity in the appended claims. The present invention, both as to its organization and manner of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accom- -panying drawings in which:

Figs. 1A and 1B illustrate schematically the upper and lower portions, respectively, of neutron well logging apparatus embodying the present invention;

Figs. 2A and l2B represent in longitudinal section the upper and lower portions of the neutron generator shown in Fig. 1B, but drawn to an enlarged scale;

Fig. 3 is a view in longitudinal section of a mounting socket which may be used for supporting the neutron generator of Figs. 2A and 2B;

A filament or cath-` Fig. 4 represents in longitudinal section another embodiment of the invention which may be incorporated in the portion of the neutron generator illustrated in Fig. 2B;

Fig. 5 represents a modilication which may be made to the apparatus of Fig. 4;

Figs. 6 and 7 are perspective views of progressive stages in the fabrication of the deuterium-emitting filaments shown in Fig. 2A;

Fig. 8 represents a modied deuterium source such as may be employed in Fig. 2A;

Fig. 9 is a view in longitudinal section of another type of deuterium source;

Fig. 10 is a yschematic diagram of an electrical circuit in which the deuterium source of Fig. 9 may be used;

Fig. l'l is a view in longitudinal section of another embodiment of the deuterium source shown in Fig. 9; and

Fig. l2 is a view in longitudinal section of a neutron generator suitable for use in the apparatus of Fig. 1B and constructed in accordance with the present invention.

In Fig. 1A of |the drawings, the neutron well logging apparatus embodying the present invention is shown disposed in a borehole 10 traversing a plurality of earth formations 11. Borehole 1@ usually contains a hydrogenous drilling liquid 12, such as water base or oil base mud, and it may be -lined with one or more strings of metallic casing (not shown) or it may be uncased as illustrated. Y

The neutron Well logging apparatus may comprise a pressure-resistant housing 13 enclosing a neutron generator 14 (Fig. 1B), a radioactivity responsive device 1S for detecting the phenomena to be observed, and associated electronic equipment required for p-roperroperation of the neutron generator and the detector, `as described in greater detail hereinafter.

A shield plate y16, disposed above detector 15, may be employed to shield the detector from radiation emanating from generator 14. If the apparatus is to be used for obtaining neutron-gamma ray logs, the shield may be composed of lead, and if neutron-neutron logs are desired, the shield may be constructed of boron carbide. Of course, a composite shield of lead and boron carbide may be utilized if both types of logs are to be made with the equipment, either successively or simultaneously.

Housing 13 is suspended in the borehole by means of an armored cable 17 which, in connection with a winch (not shown).located at the surface of the earth, is utilized to lower and raise the apparatus in the borehole in the customary manner. As Will be described later in detail cable 17 comprises a plurality of insulated conductors that electrically connect the apparatus within housing 13 with surface equipment 9.

f The neutron generator 14 (Fig. 1B) is suitably supported by a conventional shock mounting (not shown) within housing 13. The generator comprises an evacuated envelope 18, preferably `constructed of an outgassed ceramic material, such as a magnesium silicate compound, and filled with deuterium gas under a selected pressure which may be in the neighborhood of l to 10 microns of mercury.

As best seen in Fig. 2A, a metal disc 19, preferably composed of an alloy of chromium and iron, usually refered to as chrome-steel, having a temperature coefficient of expansion which substantially matches that of the ceramic material in envelope 18, divides the upper portion of the envelope into a pressure gauge section in a known manner. Alternatively, a'metallic hydride coating, such as zirconium hydride, may be fused to the ceramic and the disc silver-soldered to the coating. Of course, any well-known method may -be employed to form this metal-to-ceramic seal as well as other similar seals to be referred to hereinafter.

Supported in spaced, parallel relation below disc 19 is a ceramic partition 23 which effectively separates compartment 21 from an ion source section or compartment 24. A plurality of openings 25 annularly distributed about partition 23 communicate compartments 21 and 24. To eiect ionization of the deuterium gas, a radiofrequency coil 26 is wound about envelope :18 in the vicinity of section 24. The envelope has a diameter which is great enough so that electrons which traverse circular paths due to the eld induced by coil 26 travel distances longer than the mean-free-path, thereby to assure ionization.

As best seen in Fig. QB, another ceramic partition 27 defines a lower extremity for ion source compartment 24 and is provided with a central opening 28 which receives a quartz sleeve 29 extending into compartment 24. The lower end of sleeve 29 is ared outwardly to form a skirt 30 which abuts against the outer surface of a hemispheric cap 31 constructed of chrome-steel. A central opening 32 in cap 31 receives a section of reduced diameter of a hollow aluminum probe 33, and the lower end of the probe is deformed so as to effect a physical and electrical connection with the cap. Probe 33 extends upwardly in coaxial relation with tube 29 and terminates below the upper end of the tube.

Hemispheric cap 31 has a diameter substantially equal to the diameter of ceramic envelope 18 and the envelope is terminated above the cap. A concentric chrome-steel sleeve 34 snugly receives envelope 18 and cap 31. It is suitably fused about its upper periphery to envelope 18 and its lower periphery is soldered or welded to cap 31. To equalize the pressure on both sides of cap 31, it is provided with a plurality of openings 35.

A cylindrical extension 36 extends downwardly from cap 31 and its lower extremity receives and is fused to a ceramic insulating ring 37 having upper and lower surfaces 38 and 39. These surfaces are provided with suitable grooves or other deformities arranged to increase the radial leak path distance between cylinder 36 and an inner metallic cylinder 40 Whose lower end is snugly received by and is fused to ring 37. Cylinder 40 preferably is constructed of a metal through which tritium ions can diffuse, such as iron, although nickel, molybdenum or platinum may be used. It extends coaxially within cylinder 36 and its upper end is terminated by an integral hemispheric cap 41 disposed in spaced, concentric relation with respect to cap 31. Cap 41 forms a target for the neutron generator, and tube 33 and target 41 constitute an accelerating gap when suitably energized.

Obviously, the accelerating gap is exposed to the gas within envelope 1S. To reduce the possibility of ionization of deuterium when the electrodes of the gap are energized for accelerating deuterium ions, the spacing. between caps 31, 41 is made smaller than the mean-freepath of electrons traversing. the envelope.

It should be noted that by reason of the hemispheric configuration of the caps 31 and 41, the possibility of ionization and breakdown is minimized as compared with other arrangements. This desirable result obtains since the hemispheric form approaches the ideal case of concentric spherical electrodes wherein at a given gas pressure breakdown occurs at a higher voltage than for any other electrode configuration.

Member 40, 41 receives an electrical insulating material, such as ceramic insert 42, which conforms to the shape of the inner wall of member 40, 41. Insert 42 is sealed at its lower end to cylinder 40 and it is provided with an axial bore 43 having an upper section 44 of reduced diameter. A chrome-steel plate 45 is seated on a shoulder at the lower extremity of bore 43. This plate is suitably fused or sealed to insert 42 so as to retain an electrolyte which may be introduced via a metallic tube 46 that extends downwardly from the plate. A screwplug 47 closes tube 46 after introduction of the electrolyte which may be composed of a weak solution of sulphuric acid and water. The hydrogen in thev sulphuric acid or in the water is an isotope of atomic weight 3, known as tritium. However, both of these compounds may contain this isotope. In any event, tritium is present in an amount suicient so that when the electrolytic cell including liquid within bore 43, 44 and electrodes 41 and 45 is suitably energized, tritium diffuses through the upper end of the wall of target 41.

If desired, a catalyst may be provided in the electrolytic cell to cause recombination of tritium and oxygen gases which may be formed as a result of the decomposition of tritium oxide under the influence of beta rays emanating from tritium during the operation of the generator. For example, finely divided platinum, commonly referred to as platinum black, may be utilized as such a catalyst.

As seen in Fig. 2A, the upper, pressure-gauge section 20 of envelope 18 is provided with a pair of diametricallyopposed openings 50 and 51 which receive ceramic tubes 52 and 53, respectively. The outer ends of sleeves 52 and 53 essentially conform to the outer cylindrical shape of envelope 18 and they are suitably sealed to the envelope. Their inner ends are closed by respective, metallic cathode plates 54 and 55 which are fused thereto. Thus, the envelope is maintained pressure tight and cathodes 54 and 55 are supported in spaced, parallel relation on 0pposite sides of an annular metallic anode ring 56 which is electrically and physically connected to disc 19 by a metallic support rod 57.

A ring 58 of magnetic material is received by envelope 18 and is provided with openings 59 and 60 that are aligned with the openings in tubes 52 and 53. A cylindrical bar magnet 61 is received by openings 59 and extends into sleeve 52. It is maintained in electrical contact with plate 54 by means of a compression spring 62 and a cooperating retaining screw 63 which is threaded into opening 59. Opening and sleeve 53 are similarly provided with a bar magnet 64, and a compression spring 65 together with a retaining screw 66 maintains magnet 64 in contact with plate 55. The end poles of the magnets 61 and 64 which face one another are of opposite magnetic polarity so as to provide a magnetic eld having a component transverse to cathode plates 54 and 55 and anode 56. Ring 58 provides a return path for this magnetic field and it is also employed as an electrical terminal for plates 54 and 55. The spacing between plates 54 and 55 and the strength of the magnetic field produced by magnets 61 and 64 are arranged in a known manner to assure the occurrence of a continuous ionic discharge in the pressure-gauge section 20.

The upper extremity of envelope 1S is closed by a cup-shaped header 67 constructed of chrome-steel fused or sealed to the envelope. The header is provided with an evacuation tube 68 and electrical insulating ceramic inserts 69 and 70 through which leads 71 and 72 may be introduced into the envelope while maintaining pressuretight seals.

The portions of leads 71 and 72 extending through section 2t) are enclosed by respective glass sleeves 73 and 74 and are suitably bent or shaped so as to pass to one side of the cylinder defined by annular electrode 56. Thus, as viewed in Fig. 2A, these leads pass behind the cylinder so dened. They extend through respective electrical insulating ceramic inserts 75 and 76 in disc 19 and are connected to the ends of helical filaments 77 and 7S, respectively, which extend into envelope section 21. These filaments (to be described more particularly in connection with Figs. 6 and 7) are composed of a metallic deuteride, such as zirconium deuteride, arranged so that the pressure over their surfaces increases or decreases with temperature. Thus, at a temperature in a 7 rst temperature range, deuterium gas is emitted, while at a temperature in a second temperature range, deuterium is absorbed. To conserve power, individual, tubular heat shields 79 and 80 constructed of a reflective material of high melting point, such as tantalum or molybdenum, enclose filaments 77 and 78. The upper ends of shields '79 and 80 are soldered or welded to disc 19 and the lower ends are provided with respective closures 81 and -82 to which the lower extremities of filaments 77 and 78, respectively, are attached. Openings 83 and 84 in shields 79 and 80 communicate the compartments dened by the heat shields with compartment 21.

In constructing generator 14, the usual precautions observed in fabrication of discharge devices are observed. For example, metal materials for the various electrodes are selected so that there is relative freedom from gases that may be absorbed prior to or during the fabrication process and which may be later expelled in operation to contaminate the generator. Moreover, with the exception of the electrodes 54 and S5 of pressure gauge 20, which should be constructed of a metal that is a good secondary electron emitter, electrode metals may be selected on the basis of low secondary-electron emission characteristics to minimize the possibility of breakdown. The original outgassing is accomplished via evacuation tube 68 over which the required amount of deuteri-um is then introduced before it is sealed.

Referring now to Fig. 1A, in order to provide power for operating generator 14 and its associated circuitry, and yet remain within the voltage and current-carrying capabilities of the conductors in cable 17, power is supplied by a three-phase alternating current source 100 located at the surface of the earth. For example, the source may provide 600 volts at 400 cycles at each of its phases supplied via a three-pole, single-throw switch 101 and cable conductors 102, 103 and 104 to deltaconnected step-down transformers 105, 106 .and 107 mounted within housing 13. By utilizing transformers having a suitable stepxlown ratio, there is thus available at conductors 108, 109 and 110 a three-phase supply at 115 volts.

One phase of the supply current is applied over conductors 108, 109 to a conventional power supply 111 adapted to convert the applied alternating voltage to a higher unidirectional potential. The latter potential is supplied over conductors 112 to the anode circuit (not shown) of a radiofrequency generator 113 operating at a frequency in the range from to 100 megacycles (mcs.) per second. The radiofrequency source, in turn, is connected by conductors 115 and 116 to the end terminals of coil 26 (Fig. 1B) which isassociated with ion source 24. Filament power for the radiofrequency generator 113 is supplied over leads 117 by a step-down transformer 118 energized from another phase of the supply current available at conductors 109 and 110.

The remaining phase of supply current, available at leads 103 and 110, is fed via a voltage control circuit 119 (to be described more fully hereinafter) and lead 120 to a high voltage power supply 121. The power supply 121 may be of conventional construction or may be of the specific type described in the copending application of Wayne R. Arnold, filed March 8, 1954, bearing the Serial No. 414,761, now U.S. Patent No. 2,295,723, and assigned to the same assignee as the present application. It provides a unidirectional potential between output leads 122 and 123 in the neighborhood of 10() kilovolts. Lead 123 is the positive terminal of the supply and is grounded to the housing 13, while lead 122, the negative terminal, is connected to metallic tube 46. As .pointed out in connection with Fig. 2B, tube 46 is connected tornetallic disc 45 and the disc, in turn, is connected to target il through the impedance of the electro- 'lyte within bore 43, 44, represented in Fig. 1B schematically -by -a reistor 124 shown in dash outline. The acceased 8 celerating gap circuit is completed by a connection 126 between probe member 31, 33, 36 and housing 13.

In order. to control the neutron flux produced by generator 14, a detector in the form of a proportional counter tube 12S is disposed in housing 13 in the general vicinity of target electrode 41. The detector includes a hydrogen-fired zirconium envelope and is filled with argon gas plus a suitable impurity. Of course, other metals, such as titanium or tantalum, may be utilized as an envelope. Alternatively, the hydrogen-tired metal may be coated on the inner surface of a glass or ceramic envelope, or a ceramic envelope may be loaded with the required metallic hydride. One output lead of tube is grounded to housing 13 and the remaining output lead is connected by a lead 127 to la coupling condenser 128 (Fig. 1A), in turn, connected to a conventional pulse amplier and integrator unit 129. The necessary voltage for counter tube 12S is supplied by a conventional power supply 130, energized from power leads 109 and 110, over a lead 131 and a decoupling resistor 132. The power supply circuit is completed by a connection 133 to housing 13.

The output of unit 129, which is representative of a characteristic of the generated neutron flux, such as counts per unit time, is applied to the input circuit of a voltage control circuit 119 over a lead 134 and connections 135 and 136 to housing 13. Circuit 119 may be of conventional construction; for instance, it may include a magnetic amplifier connected in a servo circuit Whichycompares the potential from amplifier-integrator 129 with a reference potential to Vderive a control effect. This control effect may be the adjustment of the value of an impedance effectively connected between leads 110 and 120. Accordingly, the potential which energizes high voltage power supply 121 is dependent upon the generated neutron ilux so that this ux is automatically maintained at a substantially constant value.

Amplified pulses from unit 129 are supplied via a conductor 137 which extends through cable 17 to an indicator 138, such as an integrator-voltmeter, of surface equipment 9. The indicator circuit is completed by a connection 139 between housing 13 and shield .140 of the cable and a ground connection 141 at the earths surface between the shield and the indicator 13S.

In order to energize the pressure-measuring device in envelope section 20 (Fig. 1B), disc 19, to which anode 56 is connected, is connected to power supply 130 through a resistor 142 and an extension of lead 131. Magnet 58, which is connected to cathodes S4 and S5, is grounded to housing 13 by a resistor 143. The resistance value of resistor 143 is selected, in a known manner, to counteract the negative resistance characteristics of the glow discharge between anode Se and cathodes 54 and 55 of the pressure gauge, as well as to derive a voltage representing anode-cathode current. Resistor 142 interposed in lead 131 is similarly employed to counteract the effect of the negative resistance of the ionic discharge which takes place in ion source 24.

The junction of resistor 143 with the lead to ring 58 is connected by a lead y144 to one input terminal of a pressure-control circuit 145, having its other input terminal connected by a lead 146 to housing 13. Circuit may be of conventional construction comprising, for example, a magnetic servo amplifier for comparing the potential developed across resistor 143 with a reference potential. to present an impedance between output circuit leads 147 and 148 that is automatically Vcontrolled by the difference between the developed and the reference potentials.

Lead 147 is connected to supply lead 109 and lead 143 is connected to one terminal of the primary winding of a step-down transformer 149. The remaining primary terminal is connected to supply lead 110. The secondary Winding of transformer 149 is connected by leads 9 150 and 151 to parallel-connected filaments 77 and 78 via disc 19 and leads 71 and 72.

Since the potential developed at resistor 143 is a measure of the pressure in envelope 1S, this potential is also supplied by a cable conductor 152 to an indicator 153, such as a voltmeter, of surface equipment 9. If desired, a pressure-representing-potential derived in control circuit 145 may be utilized to actuate indicator 153.

A similar type of gas-pressure control system which may be utilized for regulating the pressure within envelope 18 is disclosed in the copending application of Sidney Soloway, filed December 2l, 1953, bearing the Serial No. 399,505, now U.S. Patent No. 2,880,373, and is assigned to the same assignee as the present application.

Power supply 130 also provides a voltage for energizing units 119, 129 and 145. This voltage is supplied via a lead 154 and various extensions thereof.

The portion of the well logging apparatus thus far described relates to the generation of neutrons for irradiating formation 11. In order to obtain a log, for example of the resulting garni-na radiation, means are provided for energizing detector 15, which may be a Geiger- IMueller tube (Fig. 1B), and for recording a characteristic of its output. To this end, a source of alternating current 160 in surface equipment 9 is coupled to a transformer 161 having one terminal of its secondary winding connected to the grounded shield 14) and the other terminal connected via an isolating choke 162 to a conductor 163 of cable 17. Conductor 163 traverses housing 13 and is connected to the housing via the series-connected primary windings of transformers 164 and 165. Transformer 164 is a power transformer for a conventional power supply 166 having a connection 167 to housing 13.

Power supply 166 develops the high voltage for operating tube which is applied therto via an isolating resistor 168. The remaining terminal of tube 15 is connected by a lead 169 to housing 13. The junction of resistor 168 with the lead to tube 15 is connected by a coupling condenser 170 to the input circuit of a conventional pulse amplifier 171. The input circuit is completed by a connection 172 to the housing and a voltage of suitable magnitude for operating the amplifier is derived from power supply 166 over a lead 173. Transformer 165 is a pulse transformer to which the output signal of amplifier 171 is applied.

This output signal is derived at the surface equipment 9 by a pulse transformer 174. The primary winding of the transformer is connected to a filter including a series condenser 175 and a shunt choke 176 for attenuating voltages at the frequency of source y'160. The transformers secondary winding is connected to a conventional integrator and recording unit 177. Unit 177, for example, may comprise a capacitor for deriving a potential representing the number of pulses applied per unit time and a recording voltmeter to which this potential is applied. The recording medium of the voltmeter is displaced in a customary manner in synchronism with movement of the housing 13 through borehole 10 so that the continuous log may be obtained.

In operation, housing 13 is lowered into borehole 141 prior to the closing of switch 101. Thus, operating personnel are shielded from any dangerous radiation emanating from neutron generator 14 by the earth formations 11 and drilling fluid 12. With switch 10.1 closed, radiofrequency generator 113 is energized and its output is supplied to the coil 26 which is associated with ion source 24. In addition, high voltage power supply 121 delivers its output voltage to the accelerating gap defined by tube 33 and target 41. Moreover, a positive potential is supplied by source 130 -to anode 56 relative to the cathodes 54 and 55 of the pressure gauge 20.

The radiofrequency current in coil 26 produces a radiofrequency field, and electrons traverse circular paths due to the potential gradient along each turn of the coil. The

radiofrequency lield has a sufficiently high amplitude s as to cause electrons to undergo ionizing collisions with molecules of the deuterium gas. Thus, deuterium ions are derived and, since the process is cumulative, a continuous ionic discharge occurs in the ion source.

inasmuch as disc 19 and shields 79 and 80l are at a positive potential relative to probe 33, ions in source 24 tend to drift in toward the probe. Some of these ions pass through the opening in the probe and are thus introduced into the accelerating gap. Because of the high potential impressed between probe 31, 33 and the target 41, positive ions are accelerated to high velocities prior to striking the target. The highly accelerated positive deuterium ions thus react with the tritium in the target and neutrons of ener-gies at a level of 14 million electron volts are generated.

The beam current in the accelerating gap flows through the electrolyte within chamber 43, 44 which is represented by resistor 124. By suitably apportioning the components of the electrolyte, the apparent resistance of resistor 124 is selected so as to produce a desired voltage difference between target 41 and plate 45. In other words, the current ow between electrodes 41 and 45 of the electrolytic cell is set at a predetermined value wherein electrolysis occurs and tritium ions migrate through the electrolyte toward the inner surface of target 41. Since the target is composed of a material through which such ions may diffuse, the tritium travels to the outer surface of the target wherein reactions with accelerated deuterium ions may occur. Accordingly, as neutron generator 14 operates, tritium for target 41 is continuously brought to the surface of the target and the generator may operate over extended periods of time while experiencing no material depletion in tritium.

Positive ions upon striking target 41 may produce secondary electrons which are accelerated across the accelerating gap in a direction opposite to positive ion travel. Most of these electrons pass through probe tube 33 and traverse source 24 without collision and eventually impinge upon partition 23 which absorbs their energies by conversion to heat which is dissipated. Moreover, since partition 23 is constructed of a ceramic material, usually of low atomic weight, only soft X-rays are produced by electron bombardment. It is thus evident that the accelerated electrons are prevented from undesirably causing localized heating of electrode metals in the generator and/or the occlusion of absorbed gases. In addition, partition 23 serves to minimize recombinations of electrons and ions at the surfaces of metal disc 19 and metal shields 79 and S0 of compartment 21.

High energy neutrons emanating from target 41 irradiate formations 111 as well as detector 125. A small fraction of the fast neutrons incident on the detector produce recoil protons in its hydrogenous lining. Some of these protons cause ionization in the argon and the resulting pulses are amplified and integrated in stage 129 to develop a control potential supplied to voltage control circuit 119. If the neutron flux increases, the number of counts per second increases and the voltage control circuit reduces the voltage supply to high voltage power supply 121. Accordingly, a lower voltage is applied to the accelerating gap, thereby decreasing the neutron flux. Conversely, a decrease in the neutron iiux causes an increase in the high voltage supply to the accelerating gap. In this Way, the neutron yield remains substantially constant over a wide variety of operating conditions.

This type of neutron yield control is described in the aforementioned copending application of Wayne R. Arnold.

In the pressure control system, positive ions are created in the gap between magnets 61 and 64 by spiralling electrons which are emitted from cathodes 54 and 55 when positive ions strike these cathodes. Further electrons are emitted which, in turn, produce further positive ions and a continuous discharge occurs. Ihe resulting current is a function of the gas pressure since that pressure determines the number of positive ions which can be produced. The potential developed across resistor 143 controls pressure control circuit 14S and thus the power that is supplied to Ifilaments 77 and 78 is adjusted. Each filament may operate in a rst temperature range of approximately 300lo to 600 centigrade and control circuit 145 is arranged so that the filament temperature is increased when a decreased voltage at resistor 143 indicates a decrease in pressure within envelope 18. Conversely, the filament temperature decreases when the pressure in the envelope increases and pressure may thus be maintained constant at a desired value.

Pressure control circuit 145 further operates to provide a sucient power to filaments 77 and 78 to bring them to an operating temperature in a second range of temperature below the first-mentioned range at which they absorb deuterium. Accordingly, an increase in pressure may `be compensated and, upon a reduction in pressure, circuit 145 returns the filaments to their emitting temperatures.

The irradiation of the formations 11 by the high energy neutrons produced in generator 14, results in nuclear radiation that is incident in Geiger-Mller tube 15. This occurs in a process wherein the neutrons are slowed to energy levels low enough to permit reactions producing capture gamma rays. The detector responds to gamma rays and its output is in the form of pulses which are amplied in stage 171 before being fed to the integrator and recording unit 177 of surface equipment 9. It is, therefore, apparent that a neutron-gamma ray log is obtained. This log features information regarding the earth yformations traversed by the borehole, such as enumerated in detail in the copending application of Clark Goodman, filed March 11, 1952, bearing the Serial No. 275,931 and assigned to the same assignee as the present application.

Since automatic controls are provided for the pressure and neutron tiux, the neutron generator embodying the present invention may be reliably operated during an entire logging run. The operator need not be concerned with any critical and continuous adjustments to the equipment.

In general, by reason of the construction of generator 14, as evident in Figs. 2A and 2B, a relatively rugged device is possible. Moreover, generator 14 has a vconfiguration and is small enough so that it is adapted to the elongated, small-diameter, cylindrical housing suitable to be passed through conventional oil-field boreholes. The remainder of the borehole apparatus may be readily accommodated to the size and ruggedness specifications of borehole apparatus.

Therefore, the well logging apparatus embodying the present invention meets all requirements of logging service.

Of course, other types of logs may be derived. For example, detector tube 15 may be a proportional counter lined with hydrogenous material or a boron compound. in that way, a neutron-neutron log may be obtained. Moreover, by providing suitable detectors, both a neutrongamma ray and a neutron-neutron log may be obtained simultaneously.

If desired, generator 14 may be pulsed and the detection system associated with tube 15 gated to achieve an activation log. For example, the power supply that provides the accelerating potential for generator 14 may be arranged to deliver the high voltage in pulses, rather than at a constant value. By pulsing the neutron generator, higher peak voltage may be employed without breakdown in the accelerating gap, as contrasted with the use of a steady voltage.

if desired, a pellet (not shown) of radioactive material, such as radium, may be associated with ion source 24 to assist the initiation of ionization of gas in the source.

While a sulphuric acid electrolyte has been suggested for use in chamber 43, 44 for generating tritium, obviously other suitable electrolytes may be employed for this purpose. For example, in order to preclude the build-up of gas pressure within the electrolytic cell during electrolysis, a mixture including water in which the hydrogen is the isotope tritium, sodium iodide, starch and a buffer consisting of boric acid plus sodium borate may be employed. The electrolysis of sodium iodide produces positive sodium ions at the cathode (of target 41) of the cell which reacts with the water to produce sodium hydroxide plus positive hydrogen ions. This hydrogen (tritium) migrates into the cathode and is transferred to the outer face for reaction with accelerated deuteriurn ions.

At the anode (plate 45), iodine is liberated and absorbed by the starch. The boric acid-sodium borate buffer reacts with the sodium hydroxide which is formed so that the solution is prevented from becoming too alkaline.

if desired, a shunt resistor may be placed in parallel with the cell, represented by resistor 124 in Fig 1B. ln that way, the generation and re-supply of tritium to the target may be adjusted so that it is proportional to a fixed fraction of the target current.

The relative diameters of bores 43 and 44 of the electrolytic cell are arranged to provide a required volume of electrolyte, while preventing the diffusion of tritium through unused portions of target 41. That is, bore 43 may be suitably large so as to accommodate the required volume of electrolyte, while bore 44 may be small enough so that tritium is diffused through the relatively small area of target 41 at which the ion beam irnpinges. Accordingly, substantially all of the tritium which is thus diffused enters into deuterium-tritium reactions.

ln Fig. 3, there is illustrated a view in longitudinal cross section of a mounting socket suitable for use with the neutron generator just described. It comprises a cylindrica-l shell having an inner diameter slightly larger than the outer diameter of cylinder 36 (Fig. 2B). Shell 180 is constructed of an electrically-conductive resilient material, such as an alloy of copper and Itin commonly referred to as Phosphor-bronze and its upper extremity is provided with a plurality of longitudinal slots 181 thereby to define a plurality of gripping fingers 182 suitable for frictionally and releasably receiving cylinder 36. A ceramic insulator 183 of generally disc-like configurationis supported coaxially within shell 18@ and its upper surface 184 conforms to the configuration of the lower surface 39 of insulator 37 when the neutron generator is received by the socket of Fig. 3. Its lower surface 185 is similarly shaped so that the surface distances between shell 180 and a central opening 186 in insulator 183 is relatively large. Opening 186 has an annular shoulder at its lower end in which a conductive disc 187 is seated and secured. A connector or spring clip 188 is xed to disc 187 and includes a plurality of conductive fingers which extend upwardly through opening 186. Clip 188 is positioned to receive tube 46 of the neutron generator and a lead 189 extends from disc 187.

In order to associate neutron generator 14 with the socket of Fig. 3, it is merely inserted into the socket and a mechanical, releasable connect-ion is made between fingers -182 andthe outer surface of cylinder 36. Thus, an electrical connection may be completed to cylinder 36 via shell 180. At the same time, tube 46 is received by clip 188.

With this socket construction, the high voltages which must be employed for the accelerating gap of neutron generator 14 may be safely accommodated without danger of breakdown. `Of course, if desired, a suitable shock mounting may be provided for the socket shown in Fig. 3.

The neutron generator described hereinbefore may be modified in the manner shown in Fig. 4. While only the lower portion is illustrated in Fig. 4, which is drawn to a slightly reduced scale, it is to be understood that the remainder of the generator, above partition 27, may be as represented in Fig. 2A.

The modied generator comprises a metallic'probeI electrode 200 of cup-like conguration provided with an annular lip 201 which is sealed or fused to the lower end of envelope 18. A central, integral tube 202 extends upwardly from a fiat, disc portion 203 of electrode 200 in-coaxial alignment with opening 28 in partition 27. Tube 202 has an outer diameter substantially equal to the inner diameter of a quartz sleve 204 that receives the tube and extends upwardly into opening 28 of partition 27 to which it is secured. A plurality of openings 205 in disc 203 communicate the compartment deiined by probe 200, partition 27 and sleeve 204 with the remainder of the envelope of the neutron generator.

Another ceramic envelope portion 206 having a diameter essentially equal to the diameter of envelope 18 is sealed to the lower surface of annular lip 201 and extends downwardly to a transition section 207 that is integral with a section 208 of reduced diameter. Another transition section 209 is integral with another envelope section 210 having a diameter equal to the diameter of section 206. A cup-shaped ceramic termination 211 is provided with a cylindrical wall 212 that is coaxial with section 210. Sections 210y and 212 are received by a closely-fitting metallic sleeve 13 to which they are sealed or fused in vertically spaced relation.

An annular metallic support plate 214 is peripherally soldered or welded to the inner surface of sleeve 213 and its inner edge is soldered or welded to a toroidal target 215. Target 215 may be constructed of an alloy of nickel, cobalt, manganese and iron, commonly referred to as Kovar, and a layer 216 of zirconium, or other hydrogenabsorbing metal, is coated on the inner surface of toroid 215. Layer 216 is hydrided in a well-known manner with tritium.

Of course, the spacing between target 21S and probe tube 202 is smaller than the mean-free-path. Thus, ionization of deuterium gas does not occur although these electrodes are suitably energized for operation as an accelerating gap for positive deuterium ions by applying a potential of approximately 100 kilovolts between lip 201 and sleeves 13.

With the exception of the function of the electrolytic cell for deriving tritium, the operation of the modified generator is generally similar to the operation of the generator of Figs. 2A and 2B. Deuterium ions which enter probe tube 202 are accelerated toward target 215 which is at a high negative potential relative to the probe. A beam of ions thus traverses tube 208 and diverges toward the target to strike layer 216. The impinging ions are accelerated to velocities sufficient to produce deuterium-tritium reactions wherein neutrons are generated. An alternative target arrangement to the one shown 1n Fig. 4 is illustrated -in Fig. 5. An extension 217 of ceramic tube 208 that is bent approximately 90 to the axis of tube 208 is provided with a metallic cap 219. Cap 219 has an annular groove 220 which receives the end of tube section 217 and the tube and cap are suitably sealed to one another. The surface of cap 219 within tube section 217 is coated with a layer 221 of zirconium that is hydrided with tritium.

In operation, accelerated deuterium ions pass through tube 208, 217 and strike target layer 221, thereby generating neutrons.

Alternatively, a pulse generator may be employed to switch the beam from one target section to the other while the gamma ray detector :15 (Fig. 1B) output is simultaneously gated. The arrangement may be such that the pulses representing gamma radiation for each of the two types of reactions are supplied over conductor 163 as positive and negative pulses, respectively, to give both the deuterium-tritium and deuterium-deuterium reaction yields at the same time.

The deuterium-emitting filaments 77 and 7S of Fig. 2A may be constructed in the manner shown in Figs. 6 and 7. On a wire core or support 230 constructed of molybdenum, stainless steel, or oxide-coated steel, a pair of wires 231 and 232 are wound in parallel, interspaced relation to one another to form a composite wire 233. Wire 231 may be constructed of tungsten or molybdenum and wire 232 may be an occluder metal such as zirconium. Of course, other occluders such as titanium, tantalum, or vanadium may be used for wire 232.

The composite filament wire 233 is then wound upon a mandrel to form the spiral shown in Fig. 7. Thereafter, the wire 233 is heated in an atmosphere of hydrogen to a temperature of approximately 1000 centigrade to reorient and fix the crystalline structure of the metals. Next, Wire 233 is removed from its mandrel and placed into the envelope of the neutron generator wherein it is to be used. The envelope is evacuated and filament 233 heated to a temperature above the melting point of zirconium wire 232 in order to outgas it thoroughly. Although in this condition, the zirconium tends to iiow, by reason of the spatial relationship of wires 231 and 232, the zirconium remains essentially in its initial position. Finally, filament 233 is alternately heated and cooled in an atmosphere of deuterium to effect hydriding.

If desired, the filaments 77 and 78 of Fig. 2A, which are constructed of wire 233, may be mounted in heat shields 79 and 80 provided with an interior zirconium coating. In this way, the radiation shield simultaneously serves as a getter, as the filament operates as a source of hydrogen gas. Accordingly, improved gettering and generating characteristics of the filament and shield may be obtained, thereby to provide better control of the rate of change of envelope pressure with time.

An alternative type of deuterium source, illustrated in Fig. 8, may be mounted to disc 19 of the neutron generator illustrated in Figs. 2A and 2B. A hollow, cylindrical heat shield 235 constructed of a highly reflective metal, such as molybdenum or nickel, is secured to the lower surface of disc 19. Shield 235 is terminated by a metallic cap 236 provided with a plurality of openings 237. An annular groove 238 in cap 236 receives one end of a hollow, cylindrical container 239 constructed of a refractory material, such as crushed, sintered quartz. Container 239 extends concentrically within heat shield 235 and terminates at the lower surface of disc 19 to which it is secured. The container is filled with powdered zirconium hydride 240 containing the isotope deuterium and a heater filament 241 is wound about its outer surface. The lower end of filament 241 is connected to cap 236 and the upper end is connected to a lead 242 which extends through a ceramic insulator 243 fixed within an opening in plate 19.

Alternatively, container 239 may be constructed of crushed, sintered stainless steel or nickel. In this event, an electrically insulating coating, such as refractory aluminum oxide, may be applied to the surface of filament 241.

In operation, a voltage is applied between plate -19 and lead 242, thereby to heat filament 241 to an ope-rating temperature sufficient to bring powder 240 to a temperature at which deuterium is emitted. Filament 241 may be under the control of the pressure control circuit of Fig. lA so that automatic pressure control may be maintained within the envelope in which the modied deuterium source is mounted.

Alternatively, container 239 may be sufficiently large and filled with an occluder 240 having a large enough deuterium capacity, such as uranium deuteride, to provide a dissociation pressure in the desired pressure range. The voltage applied between lead 242 and plate 19 may be pre-set, or controlled in response to a thermostat positioned within container 239, so as to obtain an essentially constant operating temperature at which powder 240 absorbs excess deuterium in the envelope, while if the deuterium pressure within the envelope drops below a desired limit, powder 240 emits deuterium.

An electron discharge type deuterium source is illustrated in Fig. 9. It comprises a hollow, cylindrical heat shield 250 ywhich is secured to the lower surface of plate 19 and terminated by a metallic cap 251 provided with a pluarality of openings 252. insulator '253 is disposed in the upper end of heat shield 250 and includes a section of reduced diameter which extends through an opening 254 in plate 19. Insulator 253 is provided with a central opening 255 through which a metallic anode 256 extends in coaxial relation with respect to heat shield 250. The outer surface of anode 256 is coated with a layer 257 of zirconium hydride containing the isotope deuterium and a lug 258 fixed to the upper end of the anode provides the means for inaking an electrical connection thereto. A helical heater filament 259 is supported adjacent the end of anode 256 by a pair of leads 260 and '261 which extend through respective ceramic insulators 262 and 263 xed in respective openings diametrically positioned in heat shield 250. Leads 260 and 261 are directed to extend along the outer wall of the heat shield and pass through respective ceramic insulators 264 and 265 in plate 19.

An electrical circuit for associating the deuterium source of Fig. 9 with the neutron generator shown in Figs. 1A and 1B is illustrated in Fig. l0. Filament 259 is connected to the secondary winding of a step-down transformer 266 having its primary winding connected to alternating-current leads 109, '110. To place variableimpedance presented by the pressure control circuit 145 in series with the ydiode 256, 259, one terminal of circuit 145 is connected to lead 109 and the other, by lead 148 and lug 258 to anode ,256 of the deuterium-source. The anode-cathode circuit for diode 256, 259 is completed by a connection 267 extending from filament lead 260 to power lead 110.

In operation, lilarnent 2,59 is heated to a given operating ternperature and the anode-cathode current of device 256, 259 is dependent upon the impedance presented between leads 147 and 148 of the pressure control circuit 145. Since this impedance is responsive to the pressure-control potential on lead 144, it is obvious that the anode-cathode current of device 256, 259 is dependent upon the pressure-control potential. When the gas pressure Within the envelope under control decreases, the impedance between leads 147 and 148 decreases, thereby increasing the anode-cathode current of the deuterium source. As a result, anode 256 is heated to a temperature at which layer 257 emits deuterium. Conversely, an increase in pressure causes the anode-cathode current of device 256, 259 to decrease, anode 256 drops in temperature and the amount of deuterium issuing from layer 257 decreases. Thus, automatic pressure control is obtained.

Although device 256, 259 has been shown in association with a source of alternating anode-cathode potential, a unidirectional potential may be employed for this purpose. In that event, pressure control circuit 145 is arranged to present a variable resistance between leads y147 and 148 that is responsive to the pressure-control potential.

Aii alternative diode-type deuterium source is shown in Fig. 11 which may be o-perated in the same manner as the diode represented in Fig. 9. The alternative structure comprises a metallic, cylindrical envelope 268 supported in spaced parallel relation to plate 19 and provided with end caps 269 and 270 constructed of an electrically insulating material, such as a ceramic. A helical heater larnent 271 extends coaxially within envelope 258 and is supported by a pair of leads 272 and 273 which extend through end caps 269 and 270, respectively, and pass through respective ceramic insulators 274 and 275 in plate 19. Another lead 276 extends through a ceramic insulator 277 in plate 19 and is soldered or welded to the exterior surface of metallic envelope 268. A layer 278 of zirconium hydride containing the visotope A disc-shaped ceramic Y 16 deuterium is coated about the interior surface of Venvelope 268 and a plurality of openings 279 and 280 are provided in end caps 269 and 270, respectively.

The deuterium source of Fig. 11 operates in substantially the same manner as does the source shown in Fig. 9, and it may be used in association with the circuit illustrated in Fig. 10.

If desired, the electrolytic cell represented in Fig. 2B may be utilized as a deuterium source.

In Fig. 12 there is illustrated another form of neutron generator suitable for use in the apparatus ofl Figs. 1A and 1B, and represented, in part, schematically. The generator comprises ahollow, cylindrical `envelope constructed of a metal, such as chrome-steel and having an upper7 integral metallic cap 286. lts lower end is closed by an electrical insulating plug 287 which may be of a ceramic material sealed to the inner wall of the envelope. A metallic partition 288 `divides the Ainterior of the generator into an upper compartment 289 'and a lower compartment 290.

A deuterium source 291 is supported within compartment '289l by a pair of leads 292 and 293 which extend through suitably sealed insulators in upper end 286 of the generator envelope. Source 291 may be of the type illustrated in Fig. 8 and made large enough in capacity so as to operate automatically to provide an equilibrium pressure of deuterium gas within envelope 285.

Partition 288 is provided with a plurality of openings 294 `and a metallic baffle plate 295 is supported above partition 288 so as to shield source 291 from any particles, such as ions, which may traverse openings 294.

A pair of leads 296 and 297 extending through ceramic closure 287 support a hollow, cylindrical electrode 298 coaxially within compartment 290. The upper lend of electrode '298 is closed and it is provided with a `plurality of openings or perforations 299 distributed about the entirety of its cylindrical wall `and upper closure. Another pair of leads 300 and 301 which extend through ceramic closure 287 are connected to the ends of a iilament wire "302 of U-shaped configuration. The bight portion of lament 302 passes through an opening 303 in a ceramic support 304 which is received by an opening 305 in the upper end of electrode 298.

The inner wal-l of envelope 285 within compartment 290 is coated with a layer 306 of zirconium hydride containing the isotope tritium and the envelope is evacuated and filled with deuterium gas at the required pressure.

The filament portion of deuterium source 291 is heated by a battery 3017 connected to leads 292 and 293 and a high voltage source 308 which develops a potential in the neighborhood oit 50 kilovolts between a negative lead 309 and a positive lead 310 provides the accelerating Voltage for the generator. Of course, a higher accelerating potential, say kilovolts, may be employed, if desired. Negative lead 309 is connected to metal envelope 285 `and is `grounded at 311. Obviously, point 311 maybe -a point on the metal housing 13 of the generator illustrated in Figs. 1A and lB. Positive lead 310` is connected to lead 296 and thus is connected to electrode299. A voltage divider 312 is shunted across leads 309 and 310 and is provided with a tap 313 connected to ytilarnentflead 300. Tap 31B is arranged to provide a volt-age of approximately volts relative to lead 310. A filament battery 314 is connected to lament leads 300 and 301.

Although the energizing circuit `for the neutron `generator of Fig. 12 is illustrated schematically, it is to be understood that the generator may be readily incorporated in the circuit shown in Figs. 1A and 1B.

In operation, battery 314 heats filament 302 to its electron-emitting temperature and since electrode 298 is at a positive potential with respect to filament 302, electrons are drawn toward it. These electrons experience ionizing collisions with molecules of deuterium and 'a continuous ionic discharge takes place within electrode 298. Since metal envelope 285 is at a negative potential rela 17 tive to electrode 298, electrons are repelled and are not drawn toward the envelope. Consequently, ionization does not occur in the space between electrode 298 and envelope 285.

Some of the ions produced within electrode 298 pass through openings 299 and are yaccelerated toward the inner surface of envelope 28S. These ions strike layer 306 With sufficient -velocity to effect deuterium-tritium reactions in which neutrons are derived.

As explained in connection with Fig. S, deuterium source 291 operates to maintain a given denterium pressure within compartment 289 and since this compartment communicates with compartment 296 via openings 294, the selected deuterium pressure is maintained within the ion source defined by filament 302 and electrode 298, as well as within the accelerating gap defined by electrode 29S and envelope 285. l

f course, if desired, a suitable pressure-measuring device similar to the one shown in Fig. 2A may be associated with the neutron generator of Fig. l2 and employed to regni-ate the temperature of deuterium source 291.

An alternative arrangement 'for energizing filament 302 may comprise a generator connected to leads 30% and 301. The generator is mechanically connected to a driving motor by an electrical insul-ating coupling designed to withstand the accelerating potential of 50 kilovolts. Thus, the motor may be energized by the potential available at the alternating current supply leads in housing 13 (Figs. lA and 1B), and yet it is electrically isolated from the high potential diierence existing between filament 302 and the housing. This general type of filament supply circuit is shown in Igreater detail in the aforementioned application of Wayne R. Arnold.

While particular embodiments ofthe present invention have been shown and described, it is lapparent that changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the aim in the appended claims is to cover all such changes and modifications Ias -fall within the true spirit and scope of this invention.

I claim:

l. A neutron generating system comprising: an envelope containing an ionizable gas and including an ion source section and an accelerating gap section; means for effecting ionization of said gas in said ion source section and for defining a path along which substantially all ions travel for at least a portion of their trajectories; a target supported Within said accelerating gap section and including an active portion having a surface spaced from a straight line extension of said path and angularly disposed relative to a plane normal to said extension of said path; means associated with said target for supplying va selected substance thereto for reacting with ions of said ionizable gas to produce neutrons; means for accelerating ions of said ionizable gas toward said target to velocities suf-H- cient to effect neutron-producing reactions; and means associated with said envelope for controlling the pressure of said ionizable gas therein.

2. A neutron generating system comprising: an envelope containing an ionizable gas and including an ion source section and an accelerating gap section; means for effecting ionization of said gas in said ion source section; a probe electrode supported within said accelerating gap section; -a target supported within said accelerating gap section in a perdetermined spatial relationship with respect to said probe electrode and including an active portion of a thickness such that ions of a selected substance may diffuse therethrough; electrolytic cell means associated with said active portion of said target for supplying ions of said selected substance thereto for reacting with ions of said ionizable gas to produce neutrons, said target defining one electrode for said electrolytic cell and said electrolytic cell including another electrode; and means for applying a high potential between said probe electrode and said other electrode 0f said electrolytic cell thereby to provide a potential for accelerating ions of said ionizable gas toward said target to velocities sufiicient to eiiect neutron-producing reactions and to provide current flow through said electrolytic cell for the operation thereof.

3. A neutron generating system comprising: an envelope containing an ionizable gas and including an ion source section and an accelerating gap section; means for effecting ionization of said gas in said ion source section; a probe electrode supported within said accelerating gap section; a target supported 'within said accelerating gap section in a predetermined spatial relationship with respect to said probe electrode and including an active portion of a thickness such that ions of a selected substance may diffuse therethrough; electrolytic cell means associated with said active portion of said target for snpplying ions of said selected substance thereto for reacting with ions of said ionizable gas to produce neutrons, said target defining one electrode for said electrolytic cell and said electrolytic cell including another electrode; means for applying a high potential between said probe electrode and said other electrode of said electrolytic cell thereby to provide a potential for accelerating ions of said ionizable gas toward said target to velocities sufficient to effect neutron-producing reactions and to provide current flow through said electrolytic cell for the operation thereof; and a shunt impedance connected to said electrodes of said electrolytic cell and having a value providing a current in said electrolytic cell of a predetermined fractional relation to the current flow between said probe electrode and said target.

4. A target electrode assembly for a neutron generator comprising: a container of electrically insulating material having at least one opening, and filled with an electrolyte including a selected substance; a metallic closure for said opening having a thickness such that ions of said selected substance may diffuse therethrough and in contact with said electrolyte thereby to define an electrode for an electrolytic cell; and another metallic electrode supported Within said container in contact with said electrolyte.

5. A target electrode assembly for a neutron generator comprising; a container of electrically insulating material having at least one opening, land filled with an electrolyte including an isotope of the element hydrogen having an atomic weight of 3; a metallic closure for said opening having a thickness such that ions of said isotope of hydrogen may diuse therethrough and in contact with said electrolyte thereby to define an electrode for an electrolytic cell; and another metallic electrode supported within said container in contact with said electrolyte.

6. A target electrode assembly for a neutron generator comprising: a container of electrically insulating material having at least one opening, and filled with an electrolyte including 'an isotope of the element hydrogen having an atomic weight of 3; a metallic closure for said opening having a thickness such that ions of said isotope of hydrogen may diffuse therethrough in contact with said electrolyte thereby to define an electrode for an electrolytic cell; another metallic electrode supported within said container in contact with said electrolyte; and means for minimizing the build-up of gas pressure within said container.

7. A target electrode assembly for a neutron generator comprising: a container of electrically insulating material havin-g at least one opening, and filled with -an electrolyte including a compound of hydrogen and oxygen, said hydrogen being composed, at least in part, of an isotope thereof of atomic weight 3, and a catalyst for promoting recombination of said isotope of hydrogen and oxygen; a metallic closure for said opening having a thickness such that ions of said isotope of hydrogen may diffuse therethrough and in contact with said electrolyte thereby to define an electrode for an electrolytic cell; and another metallic electrode supported within said container in contact with said electrolyte.

8. An accelerating gap for a neutronV generator comprising: a probe electrode having an opening through which substantially all ions may pass along a given path; t

a target electrode including an active portion having a surface spaced from a straight line extension ofsaid path and angularly disposed relative to a plane normal to said extension of said path; and `an insulating envelope enclosing said probe electrode and said target electrode and including a constricted .bend-forming section intermediate said electrodes of a size of the order of the size of said opening providing a passage for ions.

9. An accelerating gap for a neutron generator comprising: a probe electrode havingV an opening through which substantially all ions may pass along a given path; an -annular target electrodev having an inner diameter larger than said opening and including lan active, inner portion having a surface spaced from a straight line extension of said path and angularly disposed relative to a plane normal to said extension of said path; and an insulating envelope enclosing said probe electrode and said target electrode and including a constricted, 90 bendforming section intermediate said electrodes of a'size of the order of the size of said opening providing a passage for ions. Y 'l 10. A neutron generating system comprising: an envelope containing an ionizable gas and including anion source section and an accelerating gap section; means for effecting ionization of said gas in said ion source section; a target supported within said accelerating gap section and including an active portion having a selected substance for reacting with ions of said ionizable gas to produce neutrons; means for accelerating ions of said ionizable gas toward said target to velocities su'icient to effect neutron-producing reactions; and means associated with said envelope for controlling the pressure of said ionizable gas therein including a material capable of absorbing and emitting said ionizable gas and means for heating said material to a temperature providing a dissociation pressure equal to -a desired pressure within said envelope, said material being present in a sufficient quantity to maintain a preselected gas pressure within said envelope.

11. A neutron generating system comprising: kan envelope containing an ionizable gas and including an ion source section and an accelerating gap section; means for effecting ionization of said gas in said ion source section; a target supported within said accelerating gap section and including an active portion having a selected substance for reacting with ions of said ionizable gas to produce neutrons; means for accelerating ions of said ionizable gas toward said target to velocities suflicient to effect neutron-producing reactions; and means associated with said envelope for controlling the pressure of said ionizable gas therein includ-ing an anode adapted to emit said ionizable gas, a cathode, and means for selectively establishing current flow between said anode and said cathode to heat said anode to a gas-emitting temperature. Y

12. A pressure control system for -a neutron generator including an envelope containing a selected gas comprising: an anode adapted to emit said gas in communica'tion with `said envelope; a cathode associated with said anode in communication with saidenvelope; and means for selectively establishing current flow between said anode and said cathode to heat said anode to gas-emitting temperature.

13. A control system for maintaining a desired pressure of a selected gas within a gas-tight envelope comprising: an anode adapted to emit said gas disposed within said envelope; a cathode disposed within said envelope in electron-transmitting relation withrespect to said anode; means for effecting electron current yflow of adjustable magnitude between said anode and said cathode; and means responsive to the gas-pressure within said envelope for adjusting said Velectron current selectively to heat said anode to gas-emitting temperature.

14. A control system for maintaining a desired pressure of a selected gas within a gas-tight envelope cornprising: an anode adapted to emit said gas disposed within said envelope; a cathode disposed within said envelope in electron-transmitting relation with respect to said anode; an energizing circuit for said anode and cathode including a source of potential and a variable impedance connected in series circuit relation for effecting electron current flow of adjustable magnitude between said anode and said cathode; and means responsive to the gas-pressure within said envelope for adjusting said variable impedance to regulate said electron current selectively to heat said anode to gas-emitting temperature in response to a predetermined decrease in gas pressure within said envelope.

15. A neutron generating system comprising: an envelope containing an ionizable gas and including an ion source section and an accelerating gap section; means for effecting ionization of said gas in said ion source section; a probe electrode of hemispheric configuration supported within said accelerating gap section and having a central opening through which ions of said gas may pass from said ion source section; a target of hemispheric configuration smaller than said probe electrode supported within said accelerating gap section in spaced intertting relation with respect to said probe electrode; electrolytic cell means :associated with said target for supplying a selected substance to an active portion thereof for reacting with ions of said ionizable gas to produce neutrons; means for applying a high potential between said probe electrode and said target to accelerate ions of said ionizable gas from the area of said opening in said probe electrode toward said target to velocities suiiicient to effect neutron-producing reactions; and means associated with said envelope for controlling the pressure of said ionizable gas therein.

16. A neutron generating system comprising: an envelope containing an ionizable gas and including an ion source section and an accelerating gap section; means for effecting ionization of said gas in said ion source section; a probe electrode of hemispheric configuration supported within said accelerating gap section and having a central opening through which ions of said gas may pass from said ion source section; atarget of hernispheric configuration smaller than said probe electrode supported within said accelerating gap section in spaced intertting relation with respect to said probe electrode; and means for applying a high potential between said probe electrode and said target to accelerate ions of said ionizable gas from the `area of said opening in said probe electrode toward said target to velocities sutiicient to effect neutron-producing reactions.

17. A neutron generator comprising: an enevelope; means providing ions within said envelope; a probe electrode of hemispheric configuration supported within said envelope and Vhaving a central opening through which ions may pass; a target of hemispheric configuration smaller than said probe electrode supported within said envelope in spaced intertting relation with respect to said probe electrode; and means for applying a high potential between said probe electrode and said target to accelerate at least some of said ions passing into the Vicinityl of said `opening in said probe electrode toward said target to velocities sufficient to effect neutron-producing reactions. Y

18. A neutron generator comprising kan ion source including a probe electrode having an opening through which ions may pass, a target electrode for neutron-pra ducing reactions with impinging ions from said source, and an envelope sealed with respect to said electro'des to enclose the region between said electrodes and including an elongated, constricted accelerating gap section composed oi electrically insulating material intermediate said electrodes, said envelope having an annular transition portion at one end of said constricted section in suppo'rting engagement with said probe electrode about said opening, and an annular transition portion at the other end thereof in the path of secondary electrons emitted by said target during such reactions.

19. A neutron generator comprising: an ion source section and an accelerating gap section containing an ionizable gas; means for eliecting ionization of said gas in said ion source section; a probe electrode of generally hemispheric configuration supported intermediate said ion source sectio'n and said accelerating gap section and having a central passageway extending in the direction of said ion source section through which ions of said gas may pass from said ion source section; and an electrode of generally hemispheric coniiguration smaller than said probe electrode supported within said accelerating gap sectio'n in spaced intertting relation with respect to said probe electrode with an active target electrode portion disposed centrally of said smaller electrode opposite said passageway; means for applying a high potential between said probe electrode and said smaller electrode to accelerate ions of said ionizable gas from said passageway toward said active target portion to a velocity suicient to eiect neutron-producing reactions; and an annular electrical insulator sealed with respect to said probe electrode and said smaller electrode to coniine said gas within the gap between said probe electrode and said smaller electrode.

20. A neutron generating system comprising: an envelope containing an ionizable gas and including an ion source section and an accelerating gap section; means for effecting ionization of said gas in said ion source section; a target supported Within said accelerating gap section and including an active portion having a selected substance for reacting with ions of said ionizable gas to produce neutrons; means for accelerating ions of said io'nizable gas toward said target to velocities suicient to effeet neutron-producing reactions; and means associated with said envelope for controlling the pressure of said ionizable gas therein including a material capable of absorbing and emitting said ionizable gas and means for heating said material to a temperature providing a dissociation pressure equal to a desired pressure within said envelope, said material being present in a sufiicient quantity to maintain a preselected gas pressure within said envelope; #there being a constricted portion of said envelope in said accelerating gap section.

21. A neutron generating system comprising: an envelope containing an ionizable gas and including an ion source section and `an 'accelerating gap section; means for eiecting ionization of said gas in said ion source section; a target supported within said accelerating gap section vand including an active portion having -a selected substance for reacting with ions of said ionizable gas to produce neutrons; means for accelerating ions of said ionizable gas toward said target to velocities suicient to effect neutron-producing reactions; and means associated with said envelope for controlling the pressure of said ionizable gas therein including a material capable of absorbing and emitting said ionizable gas and means for heating said material to a temperature providing a dissociation pressure equal to a desired pressure within said envelope, said material being present in a suticient quantity to maintain a preselected gas pressure within said envelope; and means in said accelerating gap section to minimize the eiect of electrons emitted from said target.

22. A neutron generating system comprising: an en velope containing an io'nizable gas Iand including an ion source section and an accelerating gap section; means for eiiecting ionization of said gas in said ion source section; a target supported within said accelerating gap section and including an active portion having a selected substance for reacting with ions of said ionizable gas to produce neutrons; means for accelerating ions of said ionizable gas toward said target to velocities suflicient to effect neutron-producing reactions including electrodes forming the accelerating gap of substantially the same shape and dierent in size, supported in a coaxially nested relationship; and means associated with said envelope for controlling the pressure of said ionizable gas therein including a material capable of absorbing and emitting said ionizable gas and means for heating said material to a temperature providing a dissociation pressure equal to a desired pressure within said envelope, said material being present in a sucient quantity to maintain a preselected gas pressure within said envelope.

23. A neutro'n generating system comprising: an envelope containing an ionizable gas and including an ion source section and an accelerating gap section; means for effecting ionization of said gap in said ion source section; a. target supported within said accelerating gap section and including an active .portion having a selected substance for reacting with ions o'f said ionizable gas to produce neutrons; means for accelerating ions of said ionizable gas toward said target to velocities suicient to eiect neutron-producing reactions including means for maintaining a linear and uniform static electric field in said accelerating gap section; and means associated with said envelope for controlling the pressure of said ionizable gas therein including a material capable of absorbing and emitting said ionizable gas and means for heating said material to -a temperature providing a dissociation pressure equal to `a desired pressure within said envelope, said material being present in a suilicient quantity to maintain a preselected gas pressure within said envelope.

References Cited in the tile of this patent UNITED STATES PATENTS 2,211,668 Penning Aug. 13, 1940 2,251,190 Kallmann July 29, 1941 2,261,596 Schutze Nov. 4, 1941 2,285,622 Slepian June 9, 1942 2,433,554 Herzog Dec. 30, 1947 2,489,436 Salisbury Nov. 29, 1949 2,508,163 Hipple May 16, 1950 2,570,158 Schissel Oct. 2, -1951 2,659,046 Arps Nov. 10, 1953 2,689,918 Youmans Sept. 21, 1954 2,712,081 Fearon et al. June 28, 1955 2,769,096 Frey Oct. 30, 1956 OTHER REFERENCES Radioactivity & Nuclear Physics, Cork, published by Van Nostrand Co., Inc., New York, 1950, Second Edition, pp. 264-268.

UNITED STATES PATENT oEEICE CETIFICATE 0F Patent No., 2,983,320 May 9,

Arthur H. Frentrop It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent. should read as 'corrected below.

Column 7, line 64, for Patent No. 2,295,723 read Patent No. 2,914,677 column 9, line 36, for "therto" read thereto column 11, line 34, for "275,931" read 275,932 column 13, line 41, for "13" read f- 213 m-; column 14, line 16, after "233" insert is column 15, line 5, for "pluarality" read plurality column 20, line 55, for "enevelopeH read -m envelope Signed and sealed this 5th day of December 1961n (SEAL) Attest:

DAVID L. LADD Commissioner of, Patents uscoMM-Dc ERNEST W. SWIDEII Attesting Officer UNITED STATES PATENT OFFICE v CERTIFICATE OF CORRECTION Patent No., $2,983,820 May 9, 1961 Arthur H. Frentrop It is hereby certified that `error appears in the above numbered patent requiring correction and that the said Letters Patent, should read as "corrected below.

Column 7, line 64, for Patent No. 2,295,723"read Patent No.. 2,914,677 column 9, line 36, for "therto" read thereto column 11, line 34, for "275,931" read -m 275,932 column 13, line 4l, for "13" read 213 column 14, line 16, after "233" insert is column 15, line 5, for "pluarality" read plurality column 20, line 55, for "enevelope" read envelope Signed and sealed this 5th day of December 1961,a

` (SEAL) Attest:

ERNEST W. SWIDER Y I DAVID L. LADD Attesting Officer C Commissioner of t Patents 

